1. Field of the Invention
The present invention is in the field of equipment used for subterranean drilling. More particularly, the present invention is directed to a rotatable electric connector through which a wireline electrical cable may be connected from the surface to an instrument or several instruments located downhole, and which permits relative rotation of a lower portion of the cable relative to its upper portion. The present invention is also directed to an assembly of components which incorporate the rotatable electric connector, and to methods of conducting drilling and subterranean measuring operations with the use of the rotatable electric connector.
2. Brief Description of the Prior Art
As is well known, equipment used for lowering a drill bit into subterranean formations, while drilling for oil, gas, minerals or geothermal resources, includes a string of drill pipes and a bottom hole assembly containing the drill bit. The drill bit may be driven either from the surface by rotating the drill string with a rotary table or by a downhole motor which is driven by drilling mud fluid pumped from the surface. Because drilling at great depth below the surface is a technologically difficult and expensive operation but of great commercial importance, a large body of technology has developed in the prior art creating various methods and devices for the actual drilling operation, and also instrumentation and methods for conducting downhole measurements and for transmitting data to the surface. More particularly, it is important to know the actual underground location and direction of the bore hole even when nominally vertical drilling is intended. In "directional drilling" however, the bore hole is intentionally deviated from the vertical, and may even include horizontally disposed sections. Instrumentation which has developed in the prior art for measuring the direction (inclination, azimuth and "tool face" direction) of the advancing bore hole, as well as surveying an existing bore hole, includes electronic instruments (steering tools) which transmit data to the surface through a single conductor electric cable (wireline), and also instruments which transmit data to the surface through pressure pulses in the drilling mud fluid. In addition to such directional guidance instruments (steering tools) there are many other instruments used in connection with drilling or surveying which require transmission of data from the bore hole to the surface.
Whereas transmitting data from the bore hole to the surface through pressure pulses in the drilling mud fluid has its advantages, a principal drawback is that data transmission is slow, typically 1 bit per second (1 bps). Data transmission on a wireline cable connecting the downhole instrument with the surface is much faster, and enables data transmission rates as fast as 5,000 bits per second. Problems encountered in the prior art regarding downhole instruments which require a wireline are related to adding drill pipe to the drill string while the wireline is in position, and to rotating the drill string from the surface while the wireline is in position.
The former problem, (that of adding additional pipe to the drill string) has been more-or-less solved in the prior art by the so-called wet connector (also known in oil field parlance as a "wet stab"). The wet connector is an underwater mateable electrical connector device, which has a lower portion including an electrical connector of male pin configuration attached to the electrical cable or wireline leading downhole to whatever instrument or instruments are connected to the cable in a downhole position relative to the wet connector. The wet connector also includes an upper portion which has a female receptacle connector of complementary configuration matching the male pin and which is capable of forming an electrical connection therewith even under water or other fluid such as drilling mud. The upper (female) portion of the underwater mateable connector is electrically and mechanically connected to the wireline which leads to the surface. It is usually also attached to a weight bar disposed immediately above the upper portion and to a centralizer bar which tends to center the underwater mateable connector within the drill pipe. The underwater mateable connector is located beneath the surface, in a vertical or substantially vertical section of the drill string and its lower (male) portion is held there and kept from falling further downhole, by an interior ledge or shelf located in the drill pipe. The underwater mateable connector is used in accordance with the prior art to sever electrical connection and remove from the drill string that portion of the wireline which is attached to the male electrical connector and which leads to the surface. After a desired operation, such as addition of a drill pipe section, or rotation of the drill string is accomplished, the upper portion of the underwater mateable connector (wet connector) is lowered back into the drill string. There the female receptacle is positioned by the force of gravity upon the male pin of the lower portion, and electrical contact of the surface with the downhole instrument or instruments is reestablished. As it is known in the art and should be apparent from the description above, the underwater mateable connector of the prior art does not enable continuous rotation of the drill pipe from the surface while the wireline is connected all the way from the surface to the downhole instrument or instruments. Consequently, the prior art does not enable continuous transmission of data from downhole instrument or instruments to the surface on a wireline while the drill string is rotated continuously, or even for a short while, even though this would be highly desirable because of the high data transmission rate achievable on a wireline. Operations where this would be particularly advantageous include using a survey or steering tool, .GAMMA. ray counting tool, chlorine logging tool, electrical resistivity sensing tool, or using a tachometer for drill motor or weight-on-bit measuring tool. The inability of the prior art to continuously use a wireline for data transmission from downhole-to-surface is particularly disadvantageous when drilling or surveying bore holes with a horizontal portion or a portion having a large horizontal component. The present invention solves this problem, and provides a rotatable electrical connector which allows rotation of the drill pipe from the surface while a wireline (electrical cable) leading from the surface to downhole instrument or instruments remain in operational contact with the surface through the wireline.